Process for producing tin plate of high corrosion resistance



TEMPERATURE "F Sept. 23, 1969 SWALHIEIM ET AL 3,468,769

PROCESS FOR PRODUCING TIN PLATE OF HIGH CORROSION mzsxsmuca Filed March15, 1967 2 Sheets-Sheet 1 j o no 0.05 550 500 new mm m szconos FIG. I

INVENTORS DONALD A. SWALHEIM ROBERT W. MACKEY BY QM ATTORNEY p 1969 o.A. SWALHEIM ETAL 3,468,769

PROCESS FOR PRODUCING TIN PLATE OF HIGH CORROSION RESISTANCE DONALD A.SWALHEIM ROBERT W. MACKEY ATTORNEY United States Patent 3,468,769PROCESS FOR PRODUCING TIN PLATE OF HIGH CORROSION RESISTANCE Donald A.Swalheim, Hockessin, and Robert W. Mackey, Wilmington, Del., assignorsto E. I. du Pont de Nemours and Company, Wilmington, Del., a corporationof Delaware Continuation-impart of application Ser. No. 317,462, Oct.21, 1963. This application Mar. 15, 1967, Ser. No. 623,401

Int. Cl. C23b /52 US. Cl. 20437 2 Claims ABSTRACT OF THE DISCLOSURE Tinplate of high corrosion resistance is produced by electrolyticallydepositing tin onto steel substrate, thereafter heating the platedsubstrate to a temperature between 520 F. and 722 F., maintaining theplated substrate at such temperature for a predetermined period of time,and thereafter rapidly quenching the heated substrate.

CROSS REFERENCES TO RELATED APPLICATIONS This application is acontinuation-in-part of US. Application Ser. No. 317,462, filed Oct. 21,1963, now abandoned, which is a continuation-in-part of US. AplicationSer. No. 295,612, filed July 15, 1963, now abandoned.

BACKGROUND OF THE INVENTION Field of the invention This inventionrelates to an improved process for the melt after-treatment ofelectrolytically produced tin plate to form a superior corrosionresistant diffusion layer of tin-iron alloy at the interface of thesteel and tin.

Description of the prior art It is known to after-treat tin plate bymelting or flowbrightening, the tin layer. This was originally done forthe purpose of brightening the tin layer to make it more attractive. Inorder to retain this brightness, and to operate at high speeds, theflow-brightened tin plate is quenched in relatively cool liquid,preferably water, as shown in Smith US. Patent 2,661,328.

It was also found that flow-brightening at a temperature of 475 C. andproviding for a delay time of 1 to 3 seconds between flow-brighteningand water quenching would improve the corrosion resistance of tin plate,as described in Frankenthal Patent 3,062,725.

The corrosion resistance of tin-plated steel is customarily measured byan accelerated test procedure consisting of measuring the electriccurrent, in microamperes/ cm. flowing between the iron-tin (Fe-Sn alloyinterlayer, between the iron and tin electroplate, and a pure tinreference electrode after hours exposure in grapefruit juice. For thistest, the tin plate is stripped off the tin-electroplated steel sampleto the iron-tin alloy layer, by making the samples anodic in a 5%aqueous solution of NaOH, with a stainless steel cathode, at a cellpotential of about 0.4 volt. When all the free tin is removed thestripping current is automatically diminished to zero. The measuredcurrent, in microamperes/cm. between the exposed alloy layer and a puretin reference electrode after 20 hours exposure in grapefruit juice isreferred to as the ATC value. So-called Grade A plate must have anaverage ATC value of less than 0.05 with 95% of all values below 0.085.Moreover, for satisfactory tin can production, the weight of the alloylayer should be between 0.10 and 0.12 pound per base box.

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SUMMARY OF THE INVENTION It is an object of this invention to produceelectrolytic tin plate on a steel substrate having an average ATC valueof 0.05 or less.

It is a further object to produce electrolytic tin plate on a steelsubstrate having a uniformly thick iron-tin alloy interface betweensteel and tin and having a high resistance to corrosion.

Other objects of the invention will appear hereinafter.

The objects of this invention may be achieved by heatingelectrolytically tin-plated steel to a temperature between 520 F. and722 F. and maintaining the structure at this temperature for a period ofup to 3 seconds, and then quenching the heated structure in water. Thetime and temperature selected are maintained within the limits definedby the cross-hatched zone of FIGURE 3 of the drawing.

BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 is a diagrammatic sideelevational view of a flow-brightening apparatus which may be used inthe practice of this invention.

FIGURES 2 and 3 are graphs showing time-temperature relationships forpracticing this invention with the cross-hatched zone shown on FIGURE 3defining the time and temperature limits for practicing the invention.

DESCRIPTION OF THE INVENTION In carrying out the process of thisinvention, tin is electrodeposited in an amount from about 0.15 to about0.75 pound per base box upon a steel substrate using either an alkalineor an acid electrolyte. A preferred process is the halogen tin processdescribed by Schweikher in US. Patent 2,407,579.

Referring now to FIGURE 1 of the drawings, a tin plated sheet 3 travelsfrom the electroplating unit (not shown) through the flow-brighteningand water quenching apparatus. The sheet 3 passes around guide rolls 4and 5 and then through a conventional heating device 6, for example, aresistance or induction heating device. In passing through the heatingdevice 6, the sheet is rapidly brought to a temperature of between 520F. and 722 F. and is maintained at this temperature for a period of upto 3 seconds as determined from the time-temperature relationship shownin FIGURE 3. The lower limit for this relationship is shown graphicallyin FIGURE 2 and for periods of time up to about 2 minutes is defined bythe equation:

in which T represents any desired temperature between 520 F. and 722 F.and 6 is the minimum delay time, in seconds. The delay time is that timeinterval which begins when the temperature of the plated steel reachesthe desired flow-brightening temperature and heating ceases andterminates upon quenching the flow-brightened plate.

Immediately after the heating and flow-brightening period, the sheet 3is passed under the sink roll 7 in the water quenching tank 8 where thesheet is substantially instantaneously brought to a temperature below200 F. Results of tests conducted on 0.75 lb. plate showing effect oftemperature and delay time on ATC values of halogen tin plate are showngraphically in FIGURE 3. The numbers shown in FIGURE 3 above and belowthe line, designated Grade A plate, represent average ATC valuesobtained from Table XIII. These results indicate that production ofGrade A plate requires a refiow temperature of at least 520 F. andmaintaining the strip at this temperature for a period of at least 1.5seconds before quenching. Correspondingly shorter delay times arerequired as the temperature is increased. For example, at about 670 F.,Grade A plate will be produced with a 3 delay time of about 0.3 second.At a temperature of about 705 F., Grade A plate will be produced with adelay time of 0.1 second or less.

As stated previously, the operable zone for time and temperature inflow-brightening the plated steel is illustrated by the cross-hatchedzone shown in FIGURE 3. At temperatures below the zone, it is impossibleto form a satisfactory iron-tin alloy by the process described herein.At elevated temperatures and when using prolonged heating periods,oxidation and buckling of the steel strip occur.

At flow-brightening temperatures below 520 F., although an FeSn alloylayer is formed above the melting point of tin, 450 R, such alloy layerhas been found to be unsatisfactory for tin plate to be used,particularly for corrosive food packing, in tin can production. Atflow-brightening temperatures below 520 F. and delay times up to about 3seconds, the alloy layer is comprised of relatively large crystals withrelatively wide spacing between crystals and the alloy thickness will beinsufficient to obtain an average ATC value as low as 0.05. It is notdesirable to maintain flow-brightening temperatures much longer than 3seconds since the alloy layer thickness formed will be too great topermit satisfactory soldering or can crimping.

With a delay time of 0.1 second, the average ATC values for 0.15, 0.25and 0.5 lb./base box tin plate reflowed at temperatures of 595 F. and650 F., as shown in Tables VIII, IX and X, below, are somewhat lowerthan the average ATC values shown for 0.75 lb. plate. The deviation orlower values is not considered of real significance since the spread ofATC values for different panels reflowed under these conditions variesto a considerable extent. Also it requires about 0.1 second for thelaboratory mechanism to release the framework before the strip isquenched in water. A very small increase in delay time could account forthe lower values as noted by the steep slopes of these curves. However,at 705 F., the average ATC values for 0.15, 0.25 and 0.5 lb. plate areessentially the same as the average values shown for 0.75 lb. plate.

With a delay time of 0.8 second, the average ATC values for the lightercoatings reflowed at a temperature of 595 F. do not deviate as widely asthe values shown with a delay time of 0.1 second. At 650 F., the ATCvalues for the lighter coatings are in close agreement with the averageATC value shown for 0.75 lb. plate.

It may be concluded from the above that the ATC values are notinfluenced to any significant degree by the thickness of the tin platewithin the range investigated.

There was no significant drop in the temperature of the strip betweenthe time the power was shut off and when the strip was quennched inwater. The temperature drop with 0.3 second delay time from atemperature of 600 F.

was in the order-of-rnagnitude range of to F. based upon decrease inmillivolts as estimated by the millivolt profile curves recorded duringtemperature measurement studies.

As previously emphasized, the temperature to which the tin-plated steelis heated and the delay time between power shut off and quenching haveprofound effects on the ATC values and weights of the alloyed tin layer.Referring to the data given in Table XIII showing average ATC values andalloy weights in pounds per base box for 0.75 lb./base box tin plateheated to a temperature of about 595 F., delay time has significantefliects on the ATC values and alloy weights. Time of heating wascontrolled at 0.3 second in obtaining the data shown in Table XIII. Witha delay time of 0.1 second, the ATC values averaged 0.259 and weight ofthe alloy was 0.086 lb./base box. With a delay time of 0.4 second, theATC values averaged 0.078 and weight of the alloy was 0.112 lb./basebox. No data are given for ATC values and alloy coating weight with adelay time of 0.3 second.

Although the mechanism of alloy formation is not fully understood, ratesof diffusion to form the FeSn alloy must be extremely rapid to explainformation of the alloy layer. Increase in delay time, in addition toforming a heavier alloy layer, may provide better coverage of the alloyover the steel surface thru re-orientation of the FeSn crystals at thesteel-alloy interface.

The following examples are given to show the high degree of corrosionresistance obtained by the practice of this invention.

In the examples, the tin electroplating, flow-brightening and ATC valuedeterminations were carried out as follows:

Low carbon steel of the type customarily used for tin plate was used forpreparation of samples.

After conventional cleaning and pickling, 2 in. x 6 in. panels weretin-plated to a thickness of 0.75, 0.50, 0.25 and 0.15 pound per basebox (a base box corresponds to 435 sq. ft. of surface area) using thehalogen tin process described in US. Patent 2,407,579.

The tin-plated samples were heated to a carefully controlled giventemperature by relatively high voltage alternating current resistanceheating and after a carefully controlled given delay time were droppedinto a water quenching bath.

After quenching, the remaining tin plate was stripped from samplesleaving the steel panels with an exposed iron-tin alloy coating. Thisstripping was done by immersing the panels in a 5% NaOH solution using aconstant voltage power supply to maintain a potential of 0.4 voltbetween the panels and a stainless steel electrode. When all free tinwas removed the stripping current automatically diminished to zero.

The stripped panels were then subjected to tests to determine ATC valuesin accordance with the test described above. The results are tabulatedbelow: In the tabulated results the ATC values are given in groups of 3panels cut adjacently from each 2 in. x 6 in. panel:

TABLE I.EFFECT OF FLOW BRIGHTENING (TEMP. CA.

460 F.) ON ATC VALUES 0F HALOGEN TIN PLATE-0.75

LBJBASE BOX [Delay time before quenching with ATC values given below]TABLE II.EFFECT OF FLOW BRIGHTENING CONDITIONS (TEMP. CA. 475 F.) ON ATCVALUES OF HALO GEN TIN PLATE0.75 LBJBASE BOX [Delay time beforequenching with ATC values given below] Example Number 3 4 5 6 7 8 DelayTime (See) 0. 4 0. 8 1. 2 2.0 3.0

Average ATC Values 0.463 0.385

TABLE III-EFFECT OF FLOW BRIGH'IENIN G CONDI' TIONS (TEMP. CA. 515 F.)ON A'IC VALUES OF HALOGEN TIN PLATE0.75 LBJBASE BOX {Delay time beforequenching with ATC values given below] TIN PLATE0.75 LBJBASE BOX [Delaytime before quenching with ATC values given below] Example Number 9 1011 12 13 14 5 Example Number 36 37 38 39 DeleyTime(Sec.) 0.1 0.4 0.3 1.22.0 3.0 De1aYTime(Se') 0.307 0.303 0.135 0.030 0.033 0. 053 8-833 38330.440 0.327 0.170 0.088 0.007 0.058 0'038 0-023 0-028 AverageATCVelnes.--. 0.430 0.323 0.133 0.143 0.070 0. 5 Average ATC Values(L038 L031 TABLE IV.EFFECT OF FLOW BRIGHTENING OONDI' TIN PLATE-41.75LBJBASE BOX [Delay time before quenching with ATC values given below]TABLE VlIL-EFFECT OF FLOW-BRIGHTENING CONDI- TIONS ON ATO VALUES OF HALOGEN TIN PLATE-COAT- ING WEIGHT ABOUT 0.15 LBJBASE BOX [Delay time beforequenching with ATO values given below] Tegnp. Ter np. Temp.ExampleNumber 15 10 17 13 10 20 595 2i 40 41 42 43 44 Delay'11me(Sec.)0.1 0.4 0.8 1.2 2-0 3.0 33.30%:03253 0.1 0.0 0.1 0.8 0.1

Average ATC Values. 0.425 0.157 0.116 0.046 0.049 0.044 Average ATCValues @212 52 (L092 1023 0,036

Example Number 21 Delay Time (See) 0. 1

Average ATG Values o Q00 c 000 o o \IDOOO cu moow 5 g N @0101 q mean: 53a:

TABLE VI.-EFFECT OF FLOW BRIGHTENING C O NDITIO NS (TEMP. 651

F.) ON ATC VALUES OF HALOGEN TIN PLATE0.75 LB/BASE BOX [Delay timebefore quenching with ATC values given below] Example Number 28 DelayTime (See) 0.1

Average ATO Values .0. 152

From the above results a mathematical equation can be derived from whichmay be calculated the relative temperature, in F., and delay time, inseconds, between the limits of 520 F. and 722 F. and between and 3seconds, to obtain any desired ATC value.

This equation describing effect of refiow conditions on ATC values oftin plate is given below:

r s r no The parameters A, B, C, m, n, and p were obtained in thefollowing fashion. First, consider the boundary condition, T :450 F. Nosignificant amount of alloy is formed until T is greater than 450 F.Therefore, at 450 F. the ATC value is a constant equal to the inherentcorrosion resistance of unheated tin-plated steel, which has beenexperimentally determined to be about 0.7. The Equation 1 reduces to thefollowing under the above conditions (T:T :450 F.):

2 ATC:e

Using an ATC value of 0.7, A is calculated to be 0.357.

The coefficients B, C and the powers m, n, 7 shown in Equation 1 accountfor the non-linear character of the functional relationship betweentemperature, delay time and ATC (refer to FIGURE 3). Values for B and mcan be calculated at the boundary condition where the delay time betweenreaching peak refiow temperature and quenching the strip in water iszero (0:0). Data shown in FIGURE 3 indicate that melting the tin-platedsteel to a temperature of about 720 F. (with 0:0) will produce Grade Aplate (ATC values of 0.05). When the temperature is about 550 F. and8:0, the ATC values are approximately 0.6.

What a value of 6:0, Equation 1 reduces to:

given above, this equation can now be solved for B and m. The calculatedvalues are:

where:

The parameters A, B and m have now been estimated. The remainingparameters, C, n and p must be estimated from the experimental data.Referring to FIGURE 3, Grade A plate should be produced under thefollowing approximate conditions:

T:700 F., 6:0.1 sec. T:600 F., 0:0.65 sec. T:520 E, 0:2.0 sec.

Three simultaneous equations using the above values can be solved toobtain the following:

C=123 n:1.8 p:1.4

The unknowns in Equation 1, describing ATC values as a ftmction of delaytime and temperature have now been estimated from the experimental data.Using these calculated values of the 6 parameters, Equation 1 reducesto:

The above-referred to equation:

is a simplified close approximation to the above Equations 1 to 5 and isalso derived from the results of the above examples as shown in thecurve illustrated in FIG- URE 2. This simplified equation solved for 0to give delay time in seconds where T is temperature in degrees F. is:

Since it is obvious that many changes and modifications can be made inthe above-described details without departing from the nature and spiritof the invention, it is to be understood that the invention is not to belimited to said details except as set forth in the appended claims.

We claim:

1. The process for improving the corrosion resistance ofelectrolytically deposited tin plate which comprises electroplating asteel substrate to provide a coating of tin thereon having a weightbetween about 0.15 to 0.75 pound per 'base box of tin plate, heating thetin plate to a tem peratul'e between 520 F. and 722 F., maintaining saidtin plate at said temperature for a period up to 3 seconds, said timeand temperature being selected from the limits defined by thecross-hatched zone of FIGURE 3 of the drawing, and thereafter rapidlyquenching the heated tin plate.

2. The process of claim 1 wherein the tin is deposited onto a steelstrip from a halogen tin electro-plating bath.

References Cited UNITED STATES PATENTS 1,776,603 9/1930 Schulte 204-37XR 3,062,725 11/ 1962 Frankenthal 204-37 3,087,871 4/1963 Kamm 204-363,174,917 3/1965 Lesney et a1 204-37 3,285,838 11/1966 Morgan et al.204-37 JOHN H. MACK, Primary Examiner W. B. VANSISE, Assistant ExaminerUS. Cl. X.R. 29-1964

